In Long Term Evolution (LTE) Release 8 (R8), a base station may configure a wireless transmit/receive unit (WTRU) with downlink (DL) and uplink (UL) resources on a single DL carrier and a single UL carrier, respectively. In LTE R8, the uplink (UL) Hybrid Automatic Repeat Request (HARQ) mechanism may perform retransmissions of missing or erroneous data packets transmitted by the WTRU. The UL HARQ functionality may span across both the physical (PHY) layer and the Medium Access Control (MAC) layer. The WTRU receives acknowledgments/negative acknowledgments (ACK/NACKs) on the physical HARQ indicator channel (PHICH). That is, the PHICH may be used by a base station for transmission of HARQ feedback, (ACK or NACK), in response to Uplink Shared Channel (UL-SCH) transmissions.
User multiplexing in LTE R8 may be performed by mapping multiple PHICHs on the same set of resource elements (REs) which constitute a PHICH group. PHICHs within the same PHICH group are separated through different orthogonal Walsh sequences. In order to lower the control signalling overhead, the PHICH index pair may be implicitly associated with the index of the lowest uplink resource block used for the corresponding physical uplink shared channel (PUSCH) transmission and the cyclic shift of the corresponding uplink demodulation reference signal. The association of PHICH resources and the cyclic shifts enable the allocation of the same time and frequency resource to several WTRUs in support of uplink Multi-User Multiple Input Multiple Output (MU-MIMO). In LTE R8, there are no ambiguities or PHICH collisions, (as a result of using the same RBs), since the downlink (DL) and uplink (UL) resources are associated with a single DL carrier and a single UL carrier, respectively.
In multi-carrier wireless systems, the WTRU may be assigned or configured with multiple component carriers (CCs), such as for example, with at least one DL component carrier and at least one UL component carrier. The WTRU may be configured to aggregate a different number of CCs of possibly different bandwidths in the UL and the DL. A one-to-one relationship between the DL CC and the UL CC may not exist in multi-carrier wireless systems. In fact, multi-carrier wireless systems may use cross-carrier scheduling, where for example, a DL CC may carry information pertinent to multiple UL CCs.
Cross-carrier scheduling may include cross-carrier PHICH resource allocation. In these systems, a PHICH may be transmitted on one DL CC that may be associated with multiple UL CCs. If the PHICH is linked to two or more PUSCHs, then application of the LTE R8 PHICH parameter selection for resources may lead to ambiguities and collisions.
PHICH collisions may also occur with respect to spatial multiplexing. In spatial multiplexing, multiple transport blocks may be transmitted using multiple antennas using an identical first physical resource block (PRB) index. In other words, multiple layers of signaling or multiple streams may be transmitted over multiple antennas using the same PRB index. Therefore, given that each stream needs a corresponding ACK/NACK, the issues related to PHICH collisions stated above with respect to multiple CCs are applicable to spatial multiplexing.